Social Aspects
Behavioural and Reproduction Aspects
The American bison spends the hours of the day resting, chewing its cud or wallowing in dirt. It is most active in the early morning and late afternoon.
Bison live in a variety of groups. Females and young males may live in groups of up to 20 bison. Males also can live in groups of up to 20 males, or they can live in smaller groups or individually. Mature males and females usually live apart except during breeding season. Each bison group has a dominant male or female.
Bison are grazers and they walk while biting off mouthfuls of grass. The grass is quickly swallowed to be later brought up for cud-chewing later in the day and at night. Their diet is 99+ % grasses as opposed to forbs. They can go a long time without water and travel long distances to find water. An adult will consume more than 30 pounds of grass (dry weight) in a day. Unlike cattle, they do not have to be fed supplementarally during the winter.
Bison, especially the large bulls, look very slow and clumsy, that’s because of their lethargic movements but they can easily outrun humans, in fact, they can run up to 40 miles per hour! They can also jump 6 feet high (vertically) and 7 feet horizontally and bison are powerful swimmers. Bison are among the most dangerous animals encountered by visitors to the various U.S. and Canadian national parks, and will attack humans if provoked.
Bulls and cows at all ages will wallow in dry areas where they can stir clouds of dust. Sometimes they will wallow in wet areas as well. Dust, which permeates the coat, probably minimizes the effects of insects. The wallowing seems to be important in grooming, sensory stimulation, alleviating skin irritation, and reproductive behaviour. Wallowing also transports soils and seeds to other areas of the prairie. The wallows also serve a purpose in the prairie ecosystem. They collect rain water and form small ponds which become available to vertebrates and invertebrates. The supply of water enhances the growth of specific vegetation needing a moist or wet habitat, such as rushes.
Bison have a keen sense of smell, but their eyesight is poor. They contact one another by uttering grunts. They may seem mild mannered, but they can be unexpectedly aggressive. Threat postures include a snort or guttural bellow with head up, mouth open, and tail erect.
Female bison live in maternal herds which include other females and their offspring. Male offspring leave their maternal herd when around three years old and will either live alone or join other males in bachelor herds. Male and female herds usually do not mingle until the breeding season. However female herds may also contain a few older males. During the breeding season, dominant bulls maintain a small harem of females for mating. Individual bulls “tend” cows until allowed to mate, by following them around and chasing away rival males. The tending bull will shield the female’s vision with his body so she will not see any other challenging males. A challenging bull may bellow or roar to get a female’s attention and the tending bull has to bellow/roar back. The most dominant bulls mate in the first 2–3 weeks of the season. More subordinate bulls will mate with any remaining oestrous cow that has not mated yet. Male bison play no part in raising the young.
Bison herds have dominance hierarchies that exist for both males and females. A bison’s dominance is related to its birth date. Bison born earlier in the breeding season are more likely to be larger and more dominant as adults. Thus bison are able to pass on their dominance to their offspring as dominant bison breed earlier in the season. In addition to dominance, the older bison of a generation also have a higher fertility rate than the younger ones. Cows nurse their calves for at least 7 or 8 months but most calves seem to be weaned before the end of their first year.